1. Field of the Invention
The present invention relates to an exhaust gas purifying system. More particularly, the present invention relates to an exhaust gas purifying system capable of effectively purifying a large amount of hydrocarbons (HC) discharged from a vehicle at a low temperature at time of starting up an engine.
2. Description of the Related Art
Heretofore, in order to purify exhaust gas from an internal combustion engine of an automobile or the like, a three-way catalyst that simultaneously performs oxidation of carbon monoxide (CO) and hydrocarbons (HC) and reduction of nitrogen oxides (NOx) has widely been used. However, at a low temperature at time of starting up the engine, the three-way catalyst is not activated because of the low temperature, and thus a large amount of cold HC discharged in this case cannot be purified.
Recent years, for the purpose of purifying such cold HC, a HC adsorbing/purifying catalyst (three-way catalyst having a HC adsorbing function) has been developed, which includes zeolite as a hydrocarbon adsorbent (HC adsorbent) and a purifying catalyst such as a three-way catalyst.
The HC adsorbing/purifying catalyst temporarily adsorbs and holds cold HC discharged in a low temperature range at the time of starting up the engine, in which the three-way catalyst is not activated. Then, the HC adsorbing/purifying catalyst gradually desorbs and even purifies the HC when the three-way catalyst is activated due to a temperature increase of the exhaust gas.
As the catalyst purifying the HC desorbed from the HC adsorbent, a catalyst obtained by mixing noble metals such as rhodium (Rh), platinum (Pt) and palladium (Pd) on the same layer and a catalyst of a multilayer structure including Rh and Pd layers have been proposed. Japanese Patent Laid-Open Publication H2-56247 (published in 1990) discloses an exhaust gas purifying catalyst including a first layer containing zeolite and a second layer on the first layer. The second layer mainly contains noble metals such as Pt, Pd and Rh.
Other three-way catalysts combined with HC adsorbents have been disclosed in Japanese Patent Laid-Open Publications H6-74019 (published in 1994), H7-144119 (published in 1995), H6-142457 (published in 1994), H5-59942 (published in 1993) and H7-102957 (published in 1995).